Since a flame-resistant fiber is excellent in heat resistance and flame resistance, it is widely utilized, for example, in a spatter sheet for protecting a human body from high-heat iron powder or a welding spark scattered at welding work and in a flameproof heat insulating material of an aircraft, and a demand for the flame-resistant fiber in those fields is increasing. In addition, the flame-resistant fiber is also important as an intermediate raw material for obtaining a carbon fiber. The carbon fiber is widely used in various utilities, for example, aviation/space aeronautical materials such as aircrafts and rockets, and sports goods such as golf shafts and fishing rods because of excellent dynamical properties, various chemical properties and lightness.
Furthermore, the carbon fiber has been recently employed in aircraft and automobile applications as well as general industrial applications such as for civil engineering, construction, pressure container and windmill blade, because of lightness, excellent mechanical properties and chemical properties of the carbon fiber.
In particular, a polyacrylonitrile (hereinafter, abbreviated to “PAN”) carbon fiber has been actively industrialized up to now due to productivity or excellent properties and quality of the carbon fiber. The PAN carbon fiber is generally obtained by subjecting a PAN precursor fiber to a flame-resistant reaction, in which the PAN precursor is heated at a temperature of 200 to 300° C. in air, so as to obtain a flame-resistant fiber, and then subjecting the flame-resistant fiber to a carbonization reaction in which the flame-resistant fiber is heated in an inert atmosphere such as nitrogen.
However, the flame-resistant reaction is an exothermal reaction in a fiber form, that is, in a solid phase state. Thus, heat is likely to be stored in the fiber, and if the flame-resistant reaction is out of the condition in which the flame-resistant reaction is stably carried out, the flame-resistant reaction becomes out of control and the carbon fiber may be damaged.
For this reason, in a process for flame retarding a PAN carbon fiber, it generally takes a long time to strictly control a process speed of a flame-resistant reaction, thereby slowly carrying out the flame-resistant reaction. However, it cannot be said that this is a sufficiently high productive process.
As a means for solving the above-described technical problem, a method for obtaining a flame-resistant fiber by flame retarding a PAN polymer and then forming the PAN polymer into a fiber or a method for obtaining a flame-resistant fiber by forming a PAN polymer into a fiber and then flame retarding the fiber has been considered.
As an example of the method for obtaining a flame-resistant fiber by flame retarding a PAN polymer and then forming the PAN polymer into a fiber, Patent Document 1 discloses a method in which acrylonitrile (hereinafter, abbreviated to “AN”) polymer powder is heated in an inert atmosphere to have a density of 1.20 g/cm3 or more and then dissolved in a solvent so as to be formed into a fiber and such a fibrous material is heated. However, in this method, the AN polymer powder which is not sufficiently flame retarded is used, and, thus, there is a great change in viscosity of the solution according to time and threads are likely to be broken. Further, a strongly acidic solvent, such as sulfuric acid, acetic acid, or the like, which can easily decompose an organic polymer is used as a solvent, and, thus, it is necessary to use an apparatus formed of a special material having corrosion resistance, which causes a problem in view of cost.
Further, Patent Document 2 discloses a method in which heated AN polymer powder and non-heated AN polymer powder are mixed and then dissolved in an acidic solvent. However, in this method, the same problems, that is, corrosion resistance of the apparatus or instability of a solution, as those of the method disclosed in Patent Document 1 remain unsolved.
Meanwhile, as a method for heating PAN in a solution, Non-Patent Document 1 discloses a method in which PAN is converted into a polymer having a cyclic structure by heating a dimethylformamide (hereinafter, abbreviated to “DMF”) solution of the PAN. However, since a polymer concentration is 0.5%, being a dilute solution, and a viscosity is too low, it is actually difficult to be shaped and molded into a fiber, and if the polymer concentration is increased, the polymer is precipitated, and, thus, the use as a solution is impossible.
Further, Non-Patent Document 2 discloses a method in which PAN is degenerated by reacting primary amine with a dimethyl sulfoxide (hereinafter, abbreviated to “DMSO”) solution of the PAN. However, this solution is provided to impart a hydrophilic property to the PAN which is not yet flame retarded.
Furthermore, Patent Document 3 discloses a method in which a DMSO solution of PAN is degenerated by a nucleophilic agent such as amine and further oxidized with an oxidizing agent so as to prepare a flame-resistant polymer. However, in preparing a flame-resistant polymer using this method, it can be seen that as flame retardation progresses, the viscosity of the solution decreases and the viscosity of the solution changes according to time, which cause instability in a spinnability in a subsequent thread-forming process, deterioration in property of obtainable flame-resistant fiber and carbon fiber, and imbalance in property between single fibers of a fiber assembly. Further, since a nitrogen-based or quinone-based compound is used as the oxidizing agent, by-products such as amine or alcohol produced from the reaction may make an undesired reaction with the flame-resistant polymer, which causes coloration of a coagulation bath in the thread-forming process.
Meanwhile, as the method for obtaining a flame-resistant fiber by forming a PAN polymer into a fiber and then flame retarding the fiber, Patent Document 4 discloses a method in which a flame-resistant fiber is prepared by treating a PAN precursor fiber with a chemical, and suggests an organic nucleophilic reagent representing an amine-based compound as a cyclization accelerator and an organic nitrogen compound representing a nitrogen-based compound as an oxidizing agent. However, in the method disclosed in Patent Document 4, the chemical cannot permeate into the precursor fiber and the flame retardation slowly progresses, and, thus, the flame retardation process requires a certain time. Therefore, this method fails to reach industrial production.
Further, Patent Document 5 discloses a method in which elemental sulfur is contained in a PAN precursor fiber and a heat treatment is performed thereto in order to solve conformational heterogeneity in a cross sectional direction of a carbon fiber caused by lack of oxygen diffusion. However, the method disclosed in Patent Document 5 fails to reach industrial production due to a problem of stability in thread formation and a problem of generation of a reducing material during calcination.